It is known that various sensors are employed in automotive vehicles and various industrial machines to detect various target parameters such as the number of revolutions, the temperature and the vibration of bearing assembly and/or any other movable component parts with a view to controlling of the machines and supervision of machine statuses. Outputs generated from those sensors are generally transmitted through a wired communication system. However, it has often been experienced that the wired communication system is available only where wiring is possible, and where no wiring is possible, a wireless sensor system that transmits sensor signals wireless in the form of wireless electromagnetic signals is available. In this wireless sensor system, a transmitter is generally equipped with a compact battery.
By way of example, as far as automotive vehicles are concerned, in order to improve the safety of automotive vehicles by means of an early detection of a tire puncture or prediction of blowout through detection of the tire pressure being reduced, it is a recent trend to impose an obligation to utilize a tire pressure sensor for detecting the decrease of the tire pressure. Most of the conventional air pressure sensors for this purpose are generally of a design in which the detection signal indicative of the tire pressure is transmitted wireless to an automotive body structure by means of an electromagnetic wave. The wireless transmitter for transmitting the detection signal wireless from the air pressure sensor to the automotive body structure is, in most cases, integrated with a compact battery built therein so that the wireless transmitter can be electrically powered.
On the other hand, the anti-lock brake system (ABS) for controlling an automotive brake system by detection of the number of revolutions of the wheel with a rotation sensor has been well known. In this anti-lock brake system, in order to avoid an automobile accident resulting from breakage of a wiring system for the rotation sensor and also to minimize the cost of assemblage, the detection signal, indicative of the number of revolutions of the automobile wheel, generated by the rotation sensor is transmitted wireless in the form of an electromagnetic wave, such as disclosed in the Japanese Laid-open Patent Publication No. 2001-151090.
The Japanese Laid-open Patent Publication No. 2002-55113, for example, discloses a rotation sensor in the form of a multipolar electric power generator. The multipolar electric power generator is utilized not only to supply an electric power both the sensor and the wireless transmitter, but also to detect the number of revolutions of the automobile wheel. According to this patent publication, no supply of an electric power from the automotive body structure to the rotation sensor is needed and, accordingly, the system as a whole can be advantageously assembled compact.
The sensor system utilizing the battery has the following problems. In the first place, since the battery has a limited life, routine battery replacement is necessary resulting in complicated and cumbersome management of the lifetime of the battery. Other problems are associated with the environmental contamination resulting from disposal of the battery. In addition, where the rotation sensor utilizing the battery is used in a wheel support bearing assembly or in detecting the tire pressure, an unbalanced wheel rotation may occur as a result of increase of the sensor weight.
On the other hand, in the rotation sensor capable of generating the electric power, the electric power is generated only when the wheel starts its rotation. Therefore, the detection of the number of revolutions of the wheel tends to be unstable when the wheel rotates at an extremely low speed approximating to a halt, although the rotation sensor works satisfactorily when the wheel rotates at a speed equal to or higher than about 10 Km/h, at which the ABS is often activated. Also, this type of the rotation sensor cannot be used to detect other target parameters such as the temperature.
As discussed above, the self-generating rotation sensor is not capable of generating or supplying an electric power in some cases. Particularly, where sensors other than a rotation-detecting sensor are utilized, it has been found difficult to secure supply of an electric power to the sensors and transmission of sensor signals from those sensors.